[PATCH net] netdevsim: Initialize all fields of ip header when building dummy sk_buff

["Nikola Z. Ivanov" <zlatistiv@gmail.com>]

Syzbot reports a KMSAN uninit-value originating from
nsim_dev_trap_skb_build, with the allocation also
being performed in the same function.

The cause of the KMSAN warning is a missing assignment of
the tos and id fields of the ip header.

Fix this by calling skb_put_zero instead of skb_put to
guarantee null initialization.
Additionally remove the now redundant zero assignments
and reorder the remaining ones so that they more closely
match the order of the fields as they appear in the ip header.

Closes: https://syzkaller.appspot.com/bug?extid=23d7fcd204e3837866ff
Fixes: da58f90f11f5 ("netdevsim: Add devlink-trap support")
Signed-off-by: Nikola Z. Ivanov <zlatistiv@gmail.com>
---
There is a very similar function in psample.c called nsim_dev_psample_skb_build
which is almost identical to nsim_dev_trap_skb_build except for the
allocation flag reflecting its non-interrupt context and the fact
it does proper initialization of all fields.
Since these 2 are almost identical would it make sense to combine them
into 1, possbly by passing the allocation flags as parameters?

Thank you in advance for reviewing and answering!

 drivers/net/netdevsim/dev.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/drivers/net/netdevsim/dev.c b/drivers/net/netdevsim/dev.c
index 1e06e781c835..64b7cc3a6575 100644
--- a/drivers/net/netdevsim/dev.c
+++ b/drivers/net/netdevsim/dev.c
@@ -829,16 +829,14 @@ static struct sk_buff *nsim_dev_trap_skb_build(void)
 	skb->protocol = htons(ETH_P_IP);
 
 	skb_set_network_header(skb, skb->len);
-	iph = skb_put(skb, sizeof(struct iphdr));
-	iph->protocol = IPPROTO_UDP;
-	iph->saddr = in_aton("192.0.2.1");
-	iph->daddr = in_aton("198.51.100.1");
-	iph->version = 0x4;
-	iph->frag_off = 0;
+	iph = skb_put_zero(skb, sizeof(struct iphdr));
 	iph->ihl = 0x5;
+	iph->version = 0x4;
 	iph->tot_len = htons(tot_len);
 	iph->ttl = 100;
-	iph->check = 0;
+	iph->protocol = IPPROTO_UDP;
+	iph->saddr = in_aton("192.0.2.1");
+	iph->daddr = in_aton("198.51.100.1");
 	iph->check = ip_fast_csum((unsigned char *)iph, iph->ihl);
 
 	skb_set_transport_header(skb, skb->len);
-- 
2.53.0